The lithium-sulphur electrochemical system has a high theoretical specific energy of 2600 Wh/kg (D. Linden, T. B. Reddy, Handbook of batteries, third ed., McGraw-Hill, New-York, 2001), and is therefore of great interest at present. Specific energy is defined as the ratio of the energy output of a cell or battery to its weight, and is expressed in Wh/kg. The term specific energy is equivalent to the term gravimetric energy density.
It has been proposed to use various materials as a depolarizer substance for the positive electrode in lithium-sulphur batteries: elemental sulphur (U.S. Pat. Nos. 5,789,108; 5,814,420), sulphur-organic compounds (U.S. Pat. No. 6,090,504), sulphur-containing polymers (U.S. Pat. Nos. 6,201,100; 6,174,621; 6,117,590), and solutions of sulphur or lithium polysulphides in aprotic electrolyte systems (Rauh R. D., Abraham K. M., Pearson G. F., Surprenant J. K., Brummer S. B.: “A lithium/dissolved sulphur battery with an organic electrolyte”, J. Electrochem. Soc. 1979, vol. 126, no. 4, pp 523-527; Yamin H., Peled E.: “Electrochemistry of a nonaqueous lithium/sulphur cell”, J. of Power Sources, 1983, vol. 9, pp 281-287).
Solutions of lithium salts in aprotic dipolar solvents (typically linear or cyclic ethers) or their mixtures have been used as electrolytes in lithium-sulphur batteries (Yamin H., Penciner J., Gorenshtain A., Elam M., Peled E.: “The electrochemical behavior of polysulphides in tetrahydrofuran”, J. of Power Sources, 1985, vol. 14, pp 129-134; Yamin H., Gorenshtein A., Penciner J., Sternberg Y., Peled E.: “Lithium sulphur battery. oxidation/reduction mechanisms of polysulphides in THF solution”, J. Electrochem. Soc., 1988, vol. 135, no. 5, pp 1045-1048; Duck-Rye Chang, Suck-Hyun Lee, Sun-Wook Kim, Hee-Tak Kim: “Binary electrolyte based on tetra(ethylene glycol) dimethyl ether and 1,3-dioxolane for lithium-sulphur battery”, J. of Power Sources, 2002, vol. 112, pp 452-460).
The practical specific energy of a typical chemical source of electric energy usually reaches 20-30% of the theoretical maximum value of the specific energy of the electrochemical system that is employed. This is because various auxiliary elements (the separator, the current collectors of the electrodes, the electrolyte and other components) of the battery contribute to its total weight in addition to the electrode depolarizers. The auxiliary elements of the battery design do not themselves take part in the electrochemical reaction itself, but are provided so as to facilitate the reaction process and to promote normal functioning of the battery.
The value of the practical specific energy for laboratory lithium-sulphur cells generally reaches only 10-15% of its theoretical value, and is typically around 250-350 Wh/kg (J. Broadhead, T. Skotheim: “A safe, fast-charge, two-volt lithium/polymer cathode ‘AA’-size cell with a greater than 250 Wh kg-1 energy density”, Journal of Power Sources, 65 (1997), 1-2, 213-218; Peled E., Gorenshtein A., Segal M., Sternberg Y.: “Rechargeable lithium-sulphur battery (extended abstract)”, J. of Power Sources, 1989, vol. 26, pp 269-271).